The structure of bone

Bone is one of the hardest types of connective tissue in the body and when fully developed is composed of water, calcium salts and organic matter. Bone tissue is a type of living tissue that is made from special cells called osteoblasts. There are two main types of bone tissue: compact and cancellous. All bones have both types of tissue, the amount being dependent on the type of bone.

Compact (dense) bone

This is the hard portion of the bone that makes up the main shaft of the long bones and the outer layer of other bones. It protects spongy bone and provides a firm framework for the bone and body. The bone cells in this type of bone (osteocytes) are located in concentric rings around a central haversian canal, through which nerves, blood and lymphatic vessels pass.

Cancellous (spongy) bone

In contrast, this is lighter in weight than compact bone. It has an open sponge-like appearance and is found at the ends of long bones or at the centre of other bones. It does not have a haversian system but consists of a web-like arrangement of spaces that are filled with red bone marrow and separated by the thin processes of bone. Blood vessels run through every layer of cancellous bone, conveying nutrients and oxygen.

Space occupied by red marrow

Spongy bone (cancellous)

Yellow marrow

Compact bone

Periosteum

Articular cartilage

Space occupied by red marrow

Spongy bone (cancellous)

Yellow marrow

Compact bone

Periosteum

Articular cartilage

Diaphysis

Epiphysis

Epiphysis

Femur

Diaphysis

Epiphysis

Epiphysis

Femur

Fig 3.1 Structure of a long bone

Bone marrow

Bones contain two types of marrow - red and yellow:

• Red marrow manufactures red blood cells. It is found at the end of long bones and at the centre of other bones of the thorax and pelvis.

• Yellow marrow is found chiefly in the central cavities of long bones.

Except for the ends that form joints, bones are covered with a thin membrane of connective tissue called the periosteum. The outer layer of the periosteum is extremely dense and contains a large number of blood vessels. The inner layer contains osteoblasts and fewer blood vessels. The periosteum provides attachment for muscles, tendons and ligaments.

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